Objectives:

The origins of the major MRSA clones are still poorly understood. Previous reports have suggested a common origin for all MRSA from a single ancestral S. aureus strain that acquired the mec complex. Recent studies have shown that some MRSA strains are very divergent, implying that mecA has been transferred among S. aureus lineages at different times in the past.

Methods:

S. aureus MLST, together with BURST analysis and SCCmec-typing, has been used to probe population biology of bacteria and to predict ancestral genotypes and evolutionary descendents within groups of related genotypes.

Results

We explored the origin and the evolution of two novel MRSA clones: the Rome and the Italian clones. The Rome clone, circulating in a Rome hospital from 1997, had a characteristic clonal type (II::NH::C) and was susceptible to erythromycin, clindamycin, spectinomycin, vancomycin and teicoplanin. During the last few years, a variant of this clone has appeared and undergone evolution, consisting in the acquisition of two copies of Tn554, the integration of the pUB110 plasmid downstream of the mecA gene with the evolution of the SCCmec type I to IA and the consequent modification of mecA polymorph II to I, maintaining the sequence type ST247 (3-3-1-12-4-4-16; CC8) and the PFGE C pattern. The acquisition of extra-resistance genes determined erythromycin, spectinomycin and clindamycin resistance; moreover, these strains showed heteroresistance to glycopeptides. On the contrary, the Italian clone has always shown the same phenotypic (susceptible to tetracycline and rifampin) and genotypic features (II-E-E; ST228: 1-4-1-4-12-24-29; SCCmec I), suggesting the immediate success of these strains in the environment.

Conclusions:

Our results unambiguously indicate that the Rome clone and its variant derive from the same MRSA clone as the ‘Archaic’, the ‘Iberian’ and the ‘Brazilian’ ones (CC8). Moreover, the ‘Italian’ clone closely correlates with that of several S. aureus including MSSA, MRSA and GISA strains (CC5), confirming the horizontal mecA transfer among different S. aureus ancestral lineages. This ancestral MRSA became the successful clone that is spreading in Italy.